So, Luffy and Nami wil be taken as prisoners. This could be the devilish sacrifice Big Mom talked about on her introduction (she was really scary when she knew Luffy arrived). It's hard to believe that she is being that nice with Sanji, just because he sit to talk with her. Normally the big and influent guys don't want to appear soft and weak, this is exactly what Big Mom would look if she spare them after what they did. She already lost one commander to Urouge and couldn't kill him, so, if she really is a yonkou, I would be really disappointed if she is that soft (however, I can see her sparing Nami because of Lola and the vivre card).